The identification of very similar items is important to many businesses. An example is in magnetic storage in which magnetic disks are used in large quantities. The magnetic disks can differ in source, batch, and specifications. Being able to identify the disks can be vital to the manufacturing process whenever head/disk assemblies (HDAs) are reworked and disks reused. Identifying disks also is important to redevelopment when disks fabricated at various design parameters undergo extensive and expensive experiments. Product identification also becomes important if an unfortunate product recall is necessary. The evolution of a disk is traceable only if it can be uniquely identified. The current disk product does not have any coding for identification. Since disk problems can be supplier, machine, or even operator related, the need for disk identification is real and immediate. Cleanliness in micro-machining also is very important for products such as magnetic disks and heads used in disk drives as any contamination is trapped inside a file and can degrade performance.
Fourteen-inch and 275-mm disks of past vintage were uniquely identifiable with both alphanumeric serial numbers and machine readable bar codes. The coding had been done with an elaborate tool which was later discarded to reduce manufacturing cost. The current disk product therefore is not coded for identification. Problems have arisen in manufacturing when disks of uncertain performance are reused in HDAS, and in development when test data of disks of different specifications cannot be interpreted because the disks have been intermixed in testers/HDAs. The only sure way to curb such problems is to permanently code the disks for identification.
A disk drive or DASD includes several disks which look similar to records used on a record player or compact disks which are used in a CD player. The disks are stacked on a spindle, much like several 45 rpm records awaiting to be played. In a disk drive, however, the disks are mounted to the spindle and spaced apart so that the separate disks do not touch each other.
The surface of each disk is uniform in appearance. However, in actuality, the surface of each disk is divided into portions where data is stored. There are a number of tracks of the disk situated in concentric circles like rings on a tree. Compact disks have tracks as do the disks in a disk drive. The tracks in either the disk drive or the compact disk essentially replace the grooves on a conventional record. Each track in a disk drive is further subdivided into a number of sectors, where each sector is essentially just one section of the circumferential track.
Disks in a disk drive are made of a variety of materials. Most commonly, the disk is made of metal or plastic. The material from which the disk is made determines how data is stored on the disk. A plastic disk, such as those used as CDs, stores data using lasers and a laser is used to read the data back. Storage of data on a metal disk entails magnetizing portions of the disk in a pattern which reflects the data.
To store data on a metal disk, the metal disk is magnetized. In order to magnetize the surface of a disk, a small ceramic slider which contains a magnetic transducer known as a write head is passed over the surface of the disk. More specifically, the write head is flown at a height of less than six millionths of an inch from the surface of the disk and is flown over the track as the write head is energized to various states causing the track below to be magnetized to represent the data to be stored.
To retrieve data stored on a magnetic disk, a ceramic slider which contains a read head is flown over the metal disk. The magnetized portions of the disk induce a current in the read head. By looking at output from the read head, the data can be reconstructed for use by the computer system.
Typically, the same ceramic slider contains both a read head and a write head.
Like a record, both sides of a disk are generally used to store data or other information necessary for the operation of the disk drive. Since the disks are held in a stack and are spaced apart from one another, both the top and the bottom surface of each disk in the stack of disks has a ceramic slider associated with each surface. This would be comparable to having a stereo that could play both sides of a record at once. In the record analogy, each side would have a stylus which played the particular side of the record.
One method for identifying a disk is to place a bar code in a bar code region on the surface or edge of the disk. A bar code region is an area on the exterior surface of an object, such as a disk, which is relatively flat and smooth. It is possible to have such a flat, bar code region on the edge of a disk, provided that the curvature of the disk is small relative to the dimensions of the bar code region. A bar code is a number of bars and spaces of varying dimensions. There are several bar code symbologies available, each using its own coding system.